Pretreatment of metal surfaces prior to paint using polyaniline particles

ABSTRACT

Embodiments of a method of pretreating a metal substrate prior to painting comprise applying a first coating solution onto the metal substrate wherein the first coating solution comprises polyaniline particles at a pH less than 7 to yield a first coating on the metal substrate, rinsing the metal substrate to remove unreacted polyaniline particles, and applying a second coating solution post-rinse which comprises at least one acid and a silane composition at a pH less than 7 to yield a second coating on the metal substrate.

TECHNICAL FIELD

The present invention is generally directed to the pretreatment of metalsurfaces, and is specifically directed to pretreating metal surfaces toyield corrosion resistance and increased paint adhesion on the metalsurface prior to painting.

BACKGROUND

One of the last steps of manufacturing a good prior to packaging iscoating; most commonly referred to as painting. In addition to theaesthetic value, coating protects the good from the elements that causecorrosion. As would be known in the industry, manufactured goods requiresurface preparation before they are subjected to a final coating stage,such as a painting stage. Surface preparation typically involvesdegreasing or cleaning and subsequent coating pretreatment steps.Finished goods (e.g., auto parts, appliance parts, furniture parts,heavy equipment) are commonly fabricated from sheet, roll, forged, cast,and/or extruded materials (e.g., steel, aluminum, zinc, zinc coated,copper, plastic). During fabrication, process fluids (e.g., buffingcompounds, coolants, greases, lubricating oils, rust inhibitors,pressworking fluids, and quench oils) are necessary to enable efficientand quality production. Cleaning is required to remove these processfluids, material fines/shavings, and other surface debris orcontaminants that are generated as a result of the manufacturingprocess. Subsequent pretreatment steps are required to ensure coatingadhesion and corrosion resistance. Corrosion remains a significantconcern when processing metal parts. In addition to degrading theaesthetic look and feel of the metal parts, it also can degrade themechanical properties and the strength of the metal parts. Consequently,pretreatment of metal substrates has been utilized to produceanti-corrosive properties and longevity of the metal substrate.

Conventional methods have utilized various pretreatment methodologies toaddress corrosion. Pretreatment with metal phosphate primers (e.g., zincor iron phosphate) have been used; however, metal phosphates areenvironmentally undesirable. Furthermore, after pretreatment with metalphosphate primer, the next steps may include a rinse using chromic acidprior to painting the metal surface. Chromium based systems, whileproviding strong anti-corrosive properties, also are not environmentallydesirable and present worker safety issues.

Accordingly, there is a need for pretreatment coatings which areenvironmentally friendly, while providing anti-corrosion benefits andenhanced paint adhesion.

SUMMARY

According to one embodiment, a method of pretreating a metal substrateprior to painting comprises applying a first coating solution onto themetal substrate wherein the first coating solution comprises polyanilineparticles at a pH less than 7 to yield a first coating on the metalsubstrate, rinsing the metal substrate to remove unreacted polyanilineparticles, and applying a second coating solution post-rinse whichcomprises at least one acid and a silane composition at a pH less than 7to yield a second coating on the metal substrate.

These and additional objects and advantages provided by the embodimentsof the present invention will be more fully understood in view of thefollowing detailed description.

DETAILED DESCRIPTION

Embodiments of a method of pretreating a metal substrate prior topainting comprise providing at least one metal substrate, applying afirst coating solution comprising polyaniline particles at a pH lessthan 7 to yield a first coating on the metal substrate, rinsing themetal substrate to remove unreacted polyaniline particles, and applyinga second coating solution post-rinse which comprises at least one acidand a silane composition at a pH less than 7 to yield a second coatingon the metal substrate.

Various metals are contemplated for use in the metal substrates of thepresent invention. In one embodiment, the metal substrate may comprisesteel, aluminum, or combinations thereof. The metal substrate must becleaned prior to the application of the first coating solution. Variouscleaning methodologies are contemplated as suitable. For example, themetal substrate may be cleaned with an alkaline detergent, includingthose made available e.g., under the names Liquid MC 726 and LiquidFerro Terj by Dubois Chemical.

The first coating solution is generally applied utilizing a liquidcarrier. The first coating solution may be applied by using an openspray system, a cabinet spray washer, a belt washer, a tumbling washer,a wand system, a garden sprayer, a pressure washer, a vibratorydeburring washer, or by simply immersing the metal part in a tankcontaining the coating composition, or by steaming a metal part with thesolution. The residence or application time of the first coatingsolution on the metal substrate prior to rinsing may vary, for example,ranging from about 15 seconds to about 5 minutes.

The first coating solution may comprise organic acids, inorganic acids,or mixtures thereof. Most importantly, to achieve maximum corrosionresistance and paint adhesion, the first coating includes a dispersionof intrinsically conductive polymers (ICP). As used herein, ICPs arepolymers with π bonded electrons, which allow a free movement ofelectrons. The ICPs can achieve conductivities in the range of about10⁻⁷ to about 500 siemens per centimeter (S/cm), or in a furtherembodiment, a preferred range is 10⁻¹ to 10² S/cm.

Suitable ICPs may include polyaniline in a substituted or unsubstitutedform or any other ICPs with similar redox properties such aspolypyrrole, polythiophene, polyethylenedioxythiophene (PEDOT) andderivatives thereof. While various ICPs are suitable, the discussionbelow will focus on polyaniline for convenience. The polyanilinedispersion can include an acid doped polyaniline composition or anundoped polyaniline. The acid doping may occur during oxidation ofaniline to polyaniline. As would be familiar to the person of ordinaryskill, the oxidation may utilize a suitable oxidizing agent, e.g.,persulfate. Various dopants are contemplated as suitable. For example,and not by way of limitation, these suitable dopants may include thefollowing: inorganic acids like hydrochloric acid, sulfuric acid, orphosphoric acids; organic acids including aliphatic acids (e.g., aceticacid), or aromatic sulfonic acids (e.g., polystyrene sulfonic acids,naphthalene sulfonic acids, dodecylbenzenesulfonic acids, ordinonylnaphthalene sulfonic acid).

After oxidation, the polyaniline polymers are generally in a powderform, which is purified and then dispersed in a solvent material tocreate the polyaniline dispersion. The solvents may include polarsolvents, nonpolar solvents, or a mixture of solvents. After dispersion,the dispersion of polyaniline may optionally be incorporated into theacid solution described above. As stated above, the acid solution maycomprise one or more acids selected from organic acids, inorganic acids,and mixtures thereof. In one embodiment, the acid solution may be amixture of organic acids, polycarboxylic acids, and inorganic acids. Forexample and not by way of limitation, the inorganic acids may comprisesulfuric acid, phosphoric acid, hydrochloric acid, nitric acid, ormixtures thereof. The organic acids, which optionally are aliphatic,polycarboxylic or aromatic, may include para-toluene sulfonic acid,acetic acid, lactic acid, propionic acid, butyric acid, citric acid,glycolic acid, oxalic acid, tartaric acid, or mixtures thereof. Theamount of acid may vary from about 0.0001 to about 15.0% by wt., or fromabout 0.0005 to about 10.0% by wt., or from about 0.0008 to about 5.0%by wt. acid. In an exemplary embodiment, the acid mixture may compriseinorganic acid, organic acid, and polycarboxylic acid, each beingpresent at an amount of 0.1 to 5%. Without being bound by theory, byusing a mixture of acids, instead of a single acid, the first coatingsolution may, in specific embodiments, provide improved adhesion,improved support of redox mechanism by redoping, and/or improvedcomplexing of polyvalent ions.

Various ranges are contemplated for the pH of the coating solution. Inone embodiment, the pH range is from about 1 to about 6, or about 1 toabout 4. While the pH for the first coating solution is typically in theacidic range, it is expected that the first coating solution would alsobe suitable in alkaline pH ranges. Moreover, the first coating step maybe conducted at room temperature or at elevated temperatures. Forexample, the first coating step may occur at a temperature of from about60° F. to about 180° F.

Additionally as stated above, the first coating solution may comprisewater, or other optional organic solvents and additives. For example,the organic solvents that may be used in this invention include, forexample, glycols like C₂ to C₈ alkylene glycols as well as ethersthereof. Other organic solvents that may be used include alkanols(including diols), xylene, toluene, pyrrolidone, andN-methylpyrrolidone. For example and not by way of limitation, theorganic solvent may comprise methanol, hexylene glycol, 1,2-propanediol,3-methoxy-3-methyl-1-butanol, dipropylene glycol, ethylene glycol,glycerine, phenoxyethanol, polyethylene glycol and mixtures thereof. Inexemplary embodiments, the organic solvent may comprise methanol,hexylene glycol, and mixtures thereof. In specific embodiments, it iscontemplated that the solvent is diluted with about 50.0 to 99.99%, orfrom about 60.0 to about 99.95%, or from about 75.0 to about 99.90% byweight water.

Various compositional amounts are contemplated for the ICP (e.g.,polyaniline) particles in the first coating solution. For example, thefirst coating solution may comprise about 0.001 to about 20% by weightpolyaniline particles, or about 0.1 to about 5% by weight of polyanilineparticles. Moreover, the polyaniline particles comprise various particlesizes. For example, the polyaniline particles may comprise a particlesize of between about 0.001 μm to about 100 μm. In one embodiment, thepolyaniline particles may include nanoparticles having a size betweenabout 0.001 μm (1 nm) to about 0.1 μm (100 nm).

The first coating solution or the sub-components are commerciallyavailable. For example, the first coating solution with dispersedpolyaniline particles may be commercially available under the Ormecon®line of products produced by Enthone®.

Without being bound by theory, the polyaniline based first coatingsolution in combination with a sealer (i.e., second coating solution) isbeneficial because the combination synergistically provides suitablepaint adhesion, while also providing excellent film formability, andanti-corrosivity.

After the application of the first coating step, the present processutilizes a rinsing step to remove any unreacted material, for example,any unreacted polyaniline particles or excess acid. Without being boundby theory, this rinsing step helps minimize undesirable side reactions.It is desirable to first bond the polyaniline to the metal surface, thenrinse off unreacted material, and then apply a second coating with acoupling agent (e.g., silane) and acid (e.g., fluorozirconic acid) ofthe second coating solution. This ensures that the final coating, whichis produced by the reaction of the bonded polyaniline with thefluorozirconic acid and silane coupling agent, is properly adhered tothe metal surface. Without a rinse step, side reactions between unbondedpolyaniline and the silane coupling agent and the fluorozirconic acidwould produce impurities. Consequently, the present inventors recognizedthat applying the polyaniline, rinsing, and then applying the secondcoating solution with acid and silane coupling agent yields a superiorprimer coating due in part to the reduction of impurities and sidereactions. In contrast to the present process of applying separate firstand second coating solutions, a single coating solution including thepolyaniline, fluorozirconic acid, and silane in the same compositionwould not yield as effective a coating due to these unwanted sidereactions.

The rinsing step may utilize any suitable solvent, for example, water orany of the organic solvents listed above. It is also contemplated thatrinsing may include cleaning materials, such as a suitable alkalinedetergent described above. The rinsing step may be conducted at roomtemperature or at elevated temperatures. For example, the rinsing mayoccur at a temperature up to about 150° F.

While it may be desirable from a cost and efficiency standpoint tominimize the number of coating or rinsing steps, it is contemplated thatthe first coating step or rinsing step may occur over one or multiplesteps or stages.

After rinsing, the second coating may be applied, which includes asilane composition which is used as a coupling agent, and an additionalacid, which is utilized to increase the corrosion resistance and paintadhesion. The coupling agent reacts: a) with the active sites on themetal surface present between the void spaces between the polyanilineparticles; and b) with the polyaniline particles.

The silane compositions are organofunctional silanes including siliconhaving bonded thereto one or more alkoxy groups and preferably oneadditional organofunctional compound such as an amino, ureido, epoxy,vinyl, cyanato, or mercapto group. One type of organofunctional silanethat may be utilized is an aminoalkoxysilane. Another type oforganofunctional silane that may be utilized is an alkoxy silane.Organofunctional silanes which treat metal surfaces are disclosed, forexample, in U.S. Pat. Nos. 6,409,874, 5,750,197; 6,534,187; and6,270,884, the disclosures of which are hereby incorporated by referencein their entirety.

Suitable aminosilanes include gamma aminopropyltriethoxysilane,aminopropyltrimethoxysilane, aminoethylaminopropyltrimethoxysilane,aminoethylaminopropyltriethoxysilane, as well as bis-aminosilanes. Asuitable mercaptosilane is gamma mercaptopropyltrimethoxysilane. Othersilanes include gamma ureidopropyltrialkoxysilane,vinyltrimethoxysilane, vinyltriethoxysilane, vinyltriacetoxysilane,methacryloxypropyltrimethoxysilane,gammaglycidoxypropyltrimethoxysilane, as well as others. Below is listof some structural examples of aminosilanes:

H₂NCH₂CH₂CH₂CH₂CH₂CH₂NHCH₂CH₂CH₂Si(OCH₃)₃

H₂NCH₂CH₂CH₂CH₂SiCH₃(OCH₂CH₃)₂

H₂NCH₂CH₂CH₂Si(OCH₂CH₃)₃

H₂NCH₂CH₂CH₂Si(OCH₃)₃

H₂NCH₂CH₂CH₂SiO_(1.5))_(n) wherein n=1 to 10

H₂NCH₂CH₂CH₂SiCH₃(OCH₃)₂

H₂NCH₂CH₂NHCH₂CH₂CH₂Si(OCH₂CH₃)₃

H₂NCH₂CH₂NHCH₂CH₂CH₂Si(OCH₃)₃

H₂NCH₂CH₂NHCH₂CH₂CH₂SiCH₃(OCH₃)₂

H₂NCH₂CH₂NHCH₂CH₂NH CH₂CH₂CH₂Si(OCH₂CH₃)₃

H₂NCH₂CH₂NHCH₂CH₂NH CH₂CH₂CH₂Si(OCH₃)₃

H₂NCH₂CH₂NHCH₂CH₂NH CH₂CH₂CH₂SiCH₃(OCH₂CH₃)₂

H₂NCH₂CH₂NHCH₂CH₂NH CH₂CH₂CH₂SiCH₃(OCH₃)₂

Suitable commercial embodiments include the Silquest® line of productsproduced by OSI Specialties. One such suitable silane is Silquest® 1100,which has the following structure:

H₂NCH₂CH₂CH₂SiCH₃ (OCH₃)₂

Regarding the amount of silane in the coating solution, it is possibleto employ from about 0.0001 to about 30.0% by wt., or from about 0.0005to about 15% by wt., or from about 0.5 to about 3% by wt. of silane. Infurther embodiments, it is also contemplated to use a silanecross-linking agent. Examples of suitable cross-linking agents arelisted in U.S. Pat. No. 6,652,977, which is incorporated by referenceherein in its entirety.

As stated above, an additional acid is utilized to further bolster thecorrosion resistance and paint adhesion provided by the polyanilineparticles. In one or more embodiments, the acid of the second coatingsolution comprises inorganic acids, organic acids, or combinationsthereof. The inorganic acid of the second coating solution may comprisea metal fluoroacid. The metal fluoroacid of the second coating solutionmay be selected from the group consisting of fluorozirconic acid,fluorotitanic acid, and combinations thereof. The application of thesecond coating solution occurs for a period of about 15 seconds to about5 minutes.

Various ranges are contemplated for the pH of the second coatingsolution. In one embodiment, the pH range is from about 1 to about 6.5,or about 3 to about 6. While the pH for the second coating solution istypically in the acidic range, it is expected that the second coatingsolution would also be suitable in alkaline pH ranges. Moreover, thesecond coating step may be conducted at room temperature or at elevatedtemperatures. For example, the second coating step may occur at atemperature of from about 60° F. to about 180° F.

In a specific embodiment, fluorozirconic acid is utilized in the secondcoating. As recognized by the present inventors, the zirconizationprocess, which is facilitated by the addition of flurozirconic acid,provides excellent paint adhesion and corrosion resistance, whileeliminating the environmental issues associated with phosphate orchromium based treatment compositions. Without being by theory, thezirconium and polyaniline work synergistically to further increase thecorrosion resistance and paint adhesion above what is achievable by thezirconium or polyaniline particles singularly. Suitable commercialembodiment s for the second coating solution are contemplated, forexample, DuraLink® 450 produced by Dubois Chemical.

Similar to the first coating solution, the second coating solution maycomprise water, or other optional organic solvents and additives. Forexample, the organic solvents that may be used in this inventioninclude, for example, glycols like C₂ to C₈ alkylene glycols as well asethers thereof. Other organic solvents that may be used include alkanols(including diols), xylene, toluene, pyrrolidone, andN-methylpyrrolidone. For example and not by way of limitation, theorganic solvent may comprise methanol, hexylene glycol or mixturesthereof, 1,2-propanediol, 3-methoxy-3-methyl-1-butanol, dipropyleneglycol, ethylene glycol, glycerine, phenoxyethanol, polyethylene glycoland mixtures thereof. In exemplary embodiments, the organic solvent maycomprise methanol, hexylene glycol, and mixtures thereof.

After the application of the second coating, it is contemplated thatthere also may be an additional rinsing step to remove any excessunreacted acidic material.

It is further noted that terms like “preferably,” “generally,”“commonly,” and “typically” are not utilized herein to limit the scopeof the claimed invention or to imply that certain features are critical,essential, or even important to the structure or function of the claimedinvention. Rather, these terms are merely intended to highlightalternative or additional features that may or may not be utilized in aparticular embodiment of the present invention.

For the purposes of describing and defining the present invention it isadditionally noted that the term “substantially” is utilized herein torepresent the inherent degree of uncertainty that may be attributed toany quantitative comparison, value, measurement, or otherrepresentation. The term “substantially” is also utilized herein torepresent the degree by which a quantitative representation may varyfrom a stated reference without resulting in a change in the basicfunction of the subject matter at issue.

Having described the invention in detail and by reference to specificembodiments thereof, it will be apparent that modifications andvariations are possible without departing from the scope of theinvention defined in the appended claims. More specifically, althoughsome aspects of the present invention are identified herein as preferredor particularly advantageous, it is contemplated that the presentinvention is not necessarily limited to these preferred aspects of theinvention.

What is claimed is:
 1. A method of pretreating a metal substrate priorto painting comprising: applying a first coating solution onto the metalsubstrate wherein the first coating solution comprises sulfuric acid,citric acid, and polyaniline particles at a pH less than 7 to yield afirst coating on the metal substrate; rinsing the metal substrate toremove unreacted polyaniline and excess acid; and applying a secondcoating solution post-rinse which comprises fluorozirconic acid andaminosilane at a pH less than 7 to yield a second coating on the metalsubstrate.
 2. The method of claim 1 further comprising cleaning themetal substrate prior to the application of the first coating solution.3. The method of claim 1 wherein the metal substrate comprise steel,aluminum, or alloys of each.
 4. The method of claim 1 wherein the acidof the second coating solution further comprises organic acids.
 5. Themethod of claim 1 wherein the second coating solution further comprisesfluorotitanic acid.
 6. The method of claim 1 wherein the first coatingsolution and the second coating solution are applied via spraying themetal substrate, immersing the metal substrate, or combinations thereof.7. The method of claim 1 wherein the application of the first coatingsolution occurs for a period of about 15 seconds to about 5 minutes. 8.The method of claim 1 wherein the application of the second coatingsolution occurs for a period of about 15 seconds to about 5 minutes. 9.The method of claim 1 wherein the first coating solution comprises 0.1to about 5% by weight polyaniline particles.
 10. The method of claim 1wherein the polyaniline particles comprise a particle size of betweenabout 0.001 μm to about 100 μm.
 11. The method of claim 1 wherein thepolyaniline particles comprise nanoparticles having a particle size ator below 0.1 μm.